Evidence supporting a relationship between stress and aging is mounting as studies have shown increased rates of dexamethasone resistance and basal hypersecretion of glucocorticoids in normal elderly humans. Although clinical studies are highly relevant, such work sometimes does not permit the specificity of variables or degree of control which may be obtained using animal models. By comparing older and prime aged female rhesus monkeys and using social manipulations that are based on routine management procedures, our series of studies is testing specific hypotheses regarding 1) the ability of old females to adapt to stress, 2) the effect of stress on the pituitary-adrenocortical axis and the immune system (using functional measures), 3) the effect of social companionship and reactivity in modulating stress and 4) the effect of HRT on immune system measures in ovariectomized females. Over the last year studies have been conducted in female rhesus macaques to assess various factors that are important to aging individuals. Differences in the immune system were found between prime and old age ovariectomized female rhesus macaques confirming age related declines in the literature using intact subjects. Hormone replacement therapy (HRT) in ovariectomized females showed a greater detrimental effect on the immune system when progesterone was given than when estrogen was given. Aging is also characterized by a decreased ability of individuals to adapt to stress which is linked to hypothalamic-pituitary-adrenal axis dysregulation. The resultant data will help elucidate the stress-related immune and endocrine changes occurring with age and the role of social support and an individual's reactivity in modulating stress.